1. Field of the Invention
This invention relates generally to an AFT (automatic fine tuning) circuit and, more particularly, is directed to an AFT circuit for use with a television receiver which utilizes an electronic tuner.
2. Description of the Prior Art
In prior art, television receivers, an example of which is shown in FIG. 1 a received video signal is passed through a high frequency amplifier 1, a mixer 2, a band pass filter 3, a video intermediate frequency amplifier 4 and a video detector 5. As shown, the television receiver further includes an AFT (automatic fine tuning) circuit 6 which supplies an A2 AFT voltage to a local oscillator 7 which controls the output from mixer 2. AFT circuit 6 includes a frequency discriminator circuit 8 which detects the frequency fluctuation in the VIF (video intermediate frequency) signal from amplifier 4 and a DC amplifier 9 which amplifies the discriminated output from frequency discriminator circuit 8 to provide the AFT voltage. The frequency drift of the output signal from local oscillator 7 is thus suppressed by the AFT voltage to automatically achieve fine tuning.
To avoid erroneous operation of the AFT circuit during start-up or the like, the AFT operation is temporaily halted when the output from the power source of the television receiver is increased, during channel switching and so on, the haulting operation being termed generally AFT defeat.
In prior art tuners of the mechanical selection type, such as of the turret or, rotary switch type, in which the tuner shaft is rotated to switch a receiving channel, the AFT defeat and the release thereof after channel switching is effected is carried out by switching a contact between its ON and OFF positions. However, such a method can not be applied to an electronic tuner in which a variable reactance element, such as a variable capacitance diode or the like, is used.
This can more readily be seen with reference to a prior art AFT circuit for an electronic tuner an example of which is shown in FIG. 2. In this circuit, transistors Q1 to Q7 form a frequency discriminator circuit 8 using a phase detector circuit of a double balanced type to which the VIF signal is supplied through input terminals 10a and 10b. More particularly, transistors Q2 and Q3 are switched at the inverse phases of the VIF signal, and the transistor pairs Q4, Q7 and Q5, Q6 are switched at the reverse phases of the VIF signal which has its series resonance frequency shifted by 90.degree. through capacitors 11a and 11b and coil 12. In this case, the series resonance frequency is selected as a video intermediate frequency f.sub.p (58.75 MH.sub.z). At output terminals 13a and 13b of frequency discriminator circuit 8, a discriminated output signal is produced which is a multiple of the VIF signal and the phase shifted VIF signal. This output signal from the frequency discriminator circuit 8 is amplified by DC amplifier 9 which includes transistors Q8, Q9, Q11 and diode Q10. Diode Q10 may be connecting the collector and base of a transistor together. The output signal from the DC amplifier 9 is supplied through a low pass filter, which is formed of resistors 14 and 15 and a capacitor 16, to an output terminal 17 of AFT circuit 6, as the AFT voltage.
Referring now to FIG. 3, there is shown therein a graphical representation of the output characteristic of frequency driscriminator circuit 8, represented by S-shaped curve 22. When the output signal, represented by curve 22, is supplied to amplifier 9, it transformed into the AFT voltage having the characteristic shown by curve 23.
However, in an electronic tuner, a plurality of variable resistors are preset in correspondence with respective channels to be received. That is, a variable resistor corresponding to each received channel is selected by a channel selection operation to produce respective tuning voltages which are used to control the frequency of the local oscillator. In FIG. 2, a variable resistor 18 is representative of one of the preset variable resistors and has a first fixed terminal 19 supplied with a DC voltage and a second fixed terminal connected to ground. The output terminal 17, at which the AFT voltage is obtained, is connected to the movable tap of the variable resistor 18 through resistors 20 and 21, and a tuning voltage VC is derived from the junction point between resistors 20 and 21.
In order to preset each of the variable resistors, transistor Q1, serving as a constant current source for frequency discriminator circuit 8, is turned OFF by an AFT switch (not shown). At this time, a reference or relative zero voltage V.sub.O is derived from a voltage source +V.sub.cc supplied through a voltage divider comprised of resistors 14 and 15, and is produced at A10 output terminal 17. During the time period in which a television broadcast wave is received, transistor Q1 is turned ON and the AFT voltage, for example, V.sub.A (refer to FIG. 3), is generated to compensate for the shift in carrier frequency of the VIF signal caused by frequency drift in the local oscillator 7 which is due to the temperature characteristic, and the like of local oscillator 7. Under the above state, a channel is selected to receive a television broadcast wave, the reference voltage V.sub.O supplied to terminal 17 equivalently changes toward AFT voltage V.sub.A with the result that an erroneous AFT operation may be effected during such change. To avoid such erroneous AFT operation, an AFT defeat pulse, which is negative for a period of time after the channel is switched is supplied to the base of transistor Q1 to turn transistor Q1, resulting in transistors Q2 to Q11 being turned OFF. Thus, the charge stored in capacitor 16 is discharged through resistor 14 or 15 so that the voltage at output terminal 17 equals reference voltage V.sub.O to avoid the above erroneous AFT operation.
Upon initiation of the AFT defeat, a time period, during which the voltage at output terminal 17 returns to reference voltage V.sub.O, is determined by the level of the AFT voltage and the time contant of resistors 14 and 15 and capacitor 16. If the resistances of resistors 14 and 15 and the capacitance of capacitor 16 are given as R14, R15 and respectively, the time constant .tau. thereof can be expressed as follows: ##EQU1## This is generally a long time, that is, greater than 100 ms. The reason is that the time constant .tau. is selected so as to sufficiently attenuate the residual FM components of the vertical and horizontal synchronizing pulses in the frequency discriminated output signal. Thus, since the time period within which the voltage at output terminal 17 returns to the reference voltage V.sub.O during AFT defeat is of a relatively long duration, an erroneous operation is likely caused. This means that after a channel is switched, there is generally a long delay before reactivation of the AFT operation.
In order to cure the above defect, another prior art AFT circuit is known and has two terminals at which differential AFT voltages respectively appear. The two terminals are short-circuited by a switching element, such as an FET or the like, to carry out the AFT defeat operation. However, if the reference voltages used in regard to the differential AFT voltages are not equal, there occurs a resultant. Accordingly, it is necessary in the operation of the circuit that a differential amplifier used to produce the differential AFT voltages have no offset or superior characteristics. Further, in order to avoid erroneous operation, the differential outputs from the differential amplifier are respectively supplied to low pass filters with the result that two capacitors are utilized, resulting in substantial delay in operating time.